The present invention, generally, relates to interface circuits providing a buffer between two circuits that have a need to communicate and, more particularly, to a circuit to interface between a small, low current device-under-test and larger test instrumentation requiring relatively high current input to function.
One need for such an interface buffer has developed in the manufacture of micro chips with multiple circuits, and another need has arisen in the industry that provides service for equipment that utilizes these very small chips. In recent years, the electronics industry has improved, grown and developed in its advance of the technology it uses, and the field of miniturization is advancing now with matching speed.
For example, one monolithic chip measuring less than one-half inch square today can store one million bits of information, and in addition, such a chip can have many electronic circuits. Placing more and more electronic circuits on smaller and smaller chips makes the task of testing such circuits approach the impossible.
A typical data processor chip has 64 pins on its terminal connector, and it is not unusual for such a chip to require 60 test instruments connected to it at the same time. For example, there are electronic test instrumentation that measure accurately plus and minus electric current, voltage, electric pulse rise and fall time, and plus and minus voltage levels at prescribed values, just to mention a few.
While the monolithic chip, as a typical device-under-test, may measure only a fraction of an inch, most of the test instrumentation are in housings that are measured in feet, because there must be, for example, meters to be viewed and switches to be manipulated, even though a test circuit itself is miniaturized. Therefore, such larger size test instrumentation makes it necessary to have longer wires connecting the device-under-test to the test instrumentation in order to perform the needed tests.
Even though the test instrumentation can readily have built in suitable circuits to compensate for any capacitance that may be reflected at its input terminals, there cannot be built into the test instrumentation circuits anything to compensate for the capacitance in the wires connecting the test instrumentation to the device-under-test. It is these "long wires" with their characteristic impedance that has presented a problem.
The transmission lines created electrically by these "long wires", particularly with co-axial cable, usually have "n" pico farads per foot of capacitance, which adds up quickly and can total 100 to 150 pico farads typical. Three or four feet of such "long wires" is all it takes.
Due to the operating currents of a typical monolithic chip, as an example of a device-under-test, being measured in micro amperes, it is absolutely necessary to use co-axial cable to connect the test instrumentation, because un-shielded wires cause errors in signal transmission due to the inductance effect at the fast signal speeds that are involved. As has been pointed out above, co-axial cable has higher capacitance than other forms of connecting wires, and since its use is dictated by operating conditions, the problem is compounded.
One solution that has been attempted in the prior art is to use a "lossey line" to connect a device-under-test to the various test instrumentation. However, using a "lossey line" with its high frequency effective low capacitance per foot, the resulting degraded signals were found to create more problems than those solved by the low capacitance of the line.
Due to the degraded signals caused by a "lossey line", the accuracy of signals is poor, they develop delays and loss of speed, and a requirement of testing at low speeds causes poor testing accuracy for such a tiny circuit as the device-under-test. A circuit such as the present device-under-test is too small and lacks sufficient power to drive a high capacitance load and, also, cannot accept reflected signals.
Therefore, the problems have remained, until the present invention which, unlike a passive "lossey line", provides an active interface between the test instrumentation and a device-under-test by providing the current needed to drive the high capacitance load while, at the same time, providing a buffer against any reflected signals.